Femtocell discovery and association in cellular networks

ABSTRACT

Embodiments of a method and apparatus for discovery and association, by a mobile station, of a femto base station from a plurality of base stations. The mobile station may select a base station for consideration for association by decoding a physical layer identifier to determine that the base station is a macro base station and select a different base station based on other considerations. Other embodiments may be described and claimed.

TECHNICAL FIELD

Various embodiments described herein relate to apparatus, systems, andmethods associated with wireless communication, including mobile stationdiscovery techniques. Some embodiments relate to broadband wirelessaccess networks that operate in accordance with one of the IEEE 802.16standards.

BACKGROUND

Femto access points or alternatively femto base stations (femto BS) arelower-power, lower-cost, lower complexity cellular network base stationsthat may be installed by the user at home or small office environmentsfor indoor coverage improvements and are typically connected throughbroadband connections such as cable modem to the operator's corenetwork.

Since the femto base stations are (or will be) densely deployed, use ofthe typical cell search and cell selection methods, which are used forinitial network entry/re-entry or handover for macro base stations incellular networks, would be cumbersome and would result in extremeoverhead in system operation and mobile station (MS) complexity andexcessive power consumption. Furthermore, private femto base stationsmay be overloaded with signaling with unauthorized mobile stationstrying to select them as target base stations for handover or systementry/re-entry.

Thus a need exists for an efficient method for femtocell discovery andassociation that would reduce the complexity and time of cell search andcell selection by mobile stations and thereby results in lower MS powerconsumption as well as helping to ensure that private femto basestations will not have to exchange signals and messages withunauthorized mobile stations thereby improving their performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a mobile station according to variousembodiments.

FIG. 2 is a positional diagram of typical base stations and a mobilestation in a wireless packet-carrying network according to variousembodiments.

FIG. 3 is a flow diagram of a discovery according to variousembodiments.

FIG. 4 is a flow diagram of a handover according to various embodiments.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustratespecific embodiments to enable those skilled in the art to practicethem. Other embodiments may incorporate structural, logical, electrical,process, and other changes. Portions and features of some embodimentsmay be included in, or substituted for, those of other embodiments.Embodiments set forth in the claims encompass all available equivalentsof those claims.

FIG. 1 is a block diagram of a mobile station 100 including multipleantennas 110 and 120, a physical layer circuit 130, a media accesscontrol (MAC) layer circuit 140 and a processing circuit 150.

Antennas 110 and 120 may comprise one or more directional oromnidirectional antennas, including, for example, dipole antennas,monopole antennas, patch antennas, loop antennas, microstrip antennas orother types of antennas suitable for transmission of RF signals. In someembodiments, instead of two or more antennas, a single antenna withmultiple apertures may be used. In these embodiments, each aperture maybe considered a separate antenna. In some MIMO (multiple input multipleoutput) embodiments, antennas 110 and 120 may be effectively separatedto take advantage of spatial diversity and the different channelcharacteristics that may result between each of antennas 110 and 120 andthe antennas of a transmitting station. In some MIMO embodiments,antennas 110 and 120 may be separated by up to 1/10 of a wavelength ormore.

In some embodiments, mobile station 100 may be configured to receiveorthogonal frequency division multiplexing (OFDM) communication signalsover a multicarrier communication channel. The OFDM signals may comprisea plurality of orthogonal subcarriers. In some of these multicarrierembodiments, mobile station 100 may be part of a wireless local areanetwork (WLAN) communication station such as a wireless access point(AP) or base station. Mobile station 100 may be configured tocommunicate in accordance with an orthogonal frequency division multipleaccess (OFDMA) technique.

In some embodiments, receiver 100 may be configured to receive signalsin accordance with IEEE 802.16 standards for wireless area networks. Insome embodiments, physical layer circuit 130 is configured to handlephysical layer communications, MAC layer circuit 140 is configured tohandle MAC layer communications and processing circuit 150 is configuredto handle higher level processing. For more information with respect toIEEE 802.16 standards, please refer to “IEEE Standards for InformationTechnology—Telecommunications and Information Exchange betweenSystems”—Local Area Networks—Specific Requirements—Part 11 “Wireless LANMedium Access Control (MAC) and Physical Layer (PHY), ISO/IEC 8802-11:1999.”

FIG. 2 is a positional diagram of typical base stations, including Macrobase stations 210 and 240, and Femto base stations 220 and 230 in awireless packet-carrying network according to various embodiments.Mobile station 200 is shown roaming through Macro base station 210coverage area 250. Femto base station coverage areas 260 and 270 arealso depicted. Mobile station 200 may be handed over to or from a Macrobase station 240 or Femto base station 220 or 230.

Femto access points or alternatively femto base stations (femto BS) 220,230 are lower-power, lower-cost, lower complexity base stations that maybe installed by the user at home or small office environments for indoorcoverage improvements and may be connected through broadband connectionssuch as cable modem to the operator's core network.

Femto BS 220 operates in licensed spectrum and is operated by thecellular network operators. They are plug-and-play devices that onceconnected to the operator's core network are automatically configuredand are self-optimized and self-organized while in operation. Femto BS220 has limited coverage and is typically used as an overlay with themacro BS 210 (Although it may operate in remote and non-overlay cases,as well). The femto BS 220 and macro BS 210 may operate on the same ordifferent Frequency Assignment (FA) or frequency layer or frequencyband. Femto BS 220 may belong to an open (accessible to all subscribers)or closed (accessible to authorized subscribers) subscriber groups.

Since the femto base stations 220 are (or will be) densely deployed, useof the typical cell search and cell selection methods, which are usedfor initial network entry/re-entry or handover for macro base stations210 in cellular networks, would be cumbersome and would result inextreme overhead in system operation and Mobile Station (MS) 200complexity and excessive power consumption. Furthermore, private Femtobase 210 stations may be overloaded with signaling with unauthorizedmobile stations trying to select them as target base stations forhandover or system entry/re-entry.

In some embodiments, an efficient method for femtocell discovery andassociation that may reduce the complexity and time of cell search andcell selection by MS 200 is provided. These embodiments may result inlower MS 200 power consumption as well as ensuring the private Femtobase stations 220 will not have to exchange signals and messages withunauthorized mobile stations thereby improving their performance.

The Femto base stations 220 may either belong to Closed SubscriberGroups (CSG) or Open Subscriber Groups (OSG). In CSG, the access andservices are restricted to authorized mobile stations. The credentialsor electronic certificates may be provided to the mobile station 200 bythe cellular system operator at the time of subscription. The servingMacro BS 210 may have the knowledge (through MS context generatedfollowing session setup) of the CSGs where the MS 200 may have access.

The OSGs, on the other hand, are publicly accessible and no specificsubscription is required. The MS may still have to be authenticated bythe operator's network.

During normal operation and as part of handover, the MS 200 may measurethe received RF signal strength from the serving and the neighboringbase stations 210, 220, 230 and 240 and selects the most viablecandidate base stations as target for handover. The measurements may beconducted on the synchronization sequences that are unique to each basestation and further carry the Cell Identification (Cell_ID) or PhysicalLayer (PHY) level identifier. The type of the cell may also becommunicated via the synchronization sequences if a hierarchicalsynchronization scheme is utilized. Since the number of bits that can becarried through the synchronization sequences are limited, additionalinformation on the cell type and other configuration restrictions arebroadcasted as part of system configuration information. In some IEEE802.16m embodiments, there are two stages of Downlink (DL)synchronization. The DL synchronization is achieved by successfulacquisition of the primary advanced preamble. The primary advancedpreamble carries information about base station type (e.g., macro BS orfemto BS), system bandwidth (e.g., 5, 10, 20 MHz) and multi-carrierconfiguration (i.e., fully configured or partially configured RFcarrier). Once the primary advanced preamble is detected, the MS 200proceeds to acquisition of the secondary advanced preambles. Thesecondary advanced preamble carries a set of 768 distinct Cell_IDs thathave been partitioned into a number of subsets where each subsetcorresponds to a certain type of base station (e.g., closed/opensubscriber group femto base stations or macro base stations).

In some embodiments, the MS 200 may acquire the synchronizationsequences, to detect the Cell_ID, followed by detection of the broadcastchannel to complete the cell selection. If the MS 200 realizes that thecell is a non-accessible Femto BS or access node, it has to restart thecell search and to select another cell. This effort would prolong theinitial network entry/re-entry and handover and may involve a greatnumber of unsuccessful trials.

Upon successful acquisition of system timing and cell identification,the MS 200 may attempt to detect and decode the system configurationinformation. This information is carried via the Superframe Headers(SFH) in some IEEE 802.16m embodiments. The superframe headers,comprising Primary and Secondary Superframe Headers (P-SFH and S-SFH),are control elements that are periodically (while a large part of thisinformation remains unchanged over a long period of time, some parts maychange more frequently) broadcasted using a robust and reliabletransmission format to ensure the information can be correctly detectedby all mobile stations in the coverage area of a base station. Thecorrect and timely detection of the system information is essential forsuccessful network entry/re-entry and handover. The S-SFH content isdivided into three sub-packets (SP1, SP2, and SP3) where sub-packetscarry essential information for various system processes such as initialnetwork entry, network re-entry, Idle-State operation, etc. according totheir respective timing sensitivity. In some IEEE 802.16m embodiments,the BS Identifier (BS-ID) may be a 48 bit MAC level identifier that isused to identify the BS and is carried through S-SFH SP1 which carriesthe 12 Least Significant Bits (LSBs) of BS-ID and S-SFH SP2 whichcarries the 36 Most Significant Bits (MSBs) of the BS-ID.

Once the system parameters are successfully acquired, the cell selectioncan be made taking certain considerations into account. For example, themobile station 200 may have a preference in selecting a specific type ofthe base station (e.g., a femtocell in indoor environment) even thoughother types of the base station may be available or the MS 200 may notbe authorized to access a group of base stations despite the fact thattheir received RF signal strength might be good.

Access State is a state where the mobile station 200 performs networkentry to the selected base station. In some IEEE 802.16m embodiments,the Access State may comprise the following procedures: 1) Initialranging and uplink synchronization, 2) Basic capability negotiation, 3)Authentication, authorization, and key exchange, and 4) Registrationwith the BS. The mobile station 200 receives specific useridentification as part of Access State procedures. The IP addressassignment may follow using appropriate procedures.

The Femto base stations belonging to an operator's network may uniquelyoperate in a different frequency assignment (FA). That is Macro BS 240and Femto BS 230 may be distinguished via the frequency band in whichthey are operating.

FIG. 3 is a flow diagram of a mobile station discovery according tovarious embodiments.

Referring now to FIG. 3, The MS begins scanning of the neighbor basestations through RF measurements, at block 300. The detection of theCell_ID helps categorizing the BS type, at block 310, and depending onthe preference of the MS, a Macro or Femto candidate is selected atblocks 320 and 340. Failure in any stage of the cell search and cellselection will result in repeating the scanning and DL synchronizationat blocks 330 and 360. The Operator-ID (i.e., the 24 Most SignificantBits of BS-ID) and LSB of the BS-ID will help the MS to determinewhether it is authorized to access to the target BS, at blocks 370 and380. The BS-ID refers to the full or part of the 48-bit BS-ID which canbe internally mapped to certain human-understandable characters/namessuch as “HOME” or “OFFICE”, etc. The MS shall be subscribed to theoperator identified by the Operator-ID and shall have the full orpartial BS-ID in its White List (i.e., a local table in the MScontaining the identities of all the CSG Femto base stations to whichthe MS is subscribed and is authorized to access), blocks 390 and 395.Success from blocks 330, 340 or 395 results in selection of the basestation and entering access state, block 350. In some embodiments, partof the BS-ID or a derivation of it may used as a common identifier forthe CSGs and may be included in the White List.

If the Femto base stations are deployed in a different FA (inter-FA),the same algorithm shall be applicable except that the MS scans adifferent frequency band and conducts RF measurements during scanning inthat frequency. The other procedures remain intact and the detected CellIDs will only belong to open and/or closed subscriber group Femto basestations.

If the Femto and Macro base stations are deployed in the same FA(intra-FA), the same algorithm shall be applicable and the MS conductsRF measurements during scanning in the same frequency. Combination ofthe inter-FA and intra-FA scanning is also possible where the neighborMacro base stations operate in the same frequency band and Femto basestations operate in a different frequency band(s).

Note that there is no uplink transmission during execution of thisalgorithm and all signals and identifiers are received and decoded viaDL transmissions that will happen regardless of the MS scanning.Therefore, no additional signaling overhead will be imposed to thetarget Femto base stations.

The MS may or may not be Femto-Aware (i.e., can distinguish femto BS orMacro BS). If the MS is not Femto-Aware or is using an older version ofair-interface protocols, the legacy network entry or re-entry orhandover procedures shall be utilized.

FIG. 4 is a flow diagram of a mobile station handover according tovarious embodiments.

Referring now to FIG. 4, The serving BS broadcasts mobile neighboradvertisement (MOB_NBR-ADV or AAI_NBR-ADV) management message at aperiodic interval to identify the network and define the characteristicsof neighbor BS to potential MS seeking initial network entry or HO,block 400. The advanced air-interface (AAI) prefix refers to IEEE802.16m MAC management messages. Since the femto base stations thatbelong to closed subscription groups are not accessible to all mobilestations in the cell and considering the broadcast nature of theMOB_NBR-ADV or AAI_NBR-ADV message, the Macro BS may refrain frombroadcasting the information of CSG Femto base stations, block 400. Thishelps reduce the size of the MOB_NBR-ADV or AAI_NBR-ADV message.

The scanning interval is defined as the time during which the MS scansfor available base stations. The mobile scanning interval request(MOB_SCN-REQ or AAI_SCN-REQ) management/control message sent by the MSin the Connected State contains a group of neighbor base stations forwhich scanning and association are requested. This message shall includethe identity (full or partial BS-ID) of the CSG Femto base stations towhich the MS is subscribed (if the MS is a Femto Preferred meaning itprefers to be associated with a Femto BS even though Macro stations maybe available in its neighborhood), block 410. Note that this is aunicast MAC management message based on which the serving BS may updatethe MS context to reflect the CSG Femto BS list that the MS may belongto.

Upon reception of MOB_SCN-REQ or AAI_SCN-REQ message by the serving BS,the BS responds with mobile scanning interval response (MOB_SCN-RSP orAAI_SCN-RSP) management/control message granting or refusing the MSscanning interval request or a different interval and further containsthe list of recommended neighbor base stations by the serving BS, atblock 420.

Following receipt of the MOB_SCN-RSP or AAI_SCN-RSP MACmanagement/control message granting the request, the MS scans for one ormore neighbor base stations and may attempt to select the cell andassociate with that cell through completion of the Access Stateprocedures. The MS may scan all the recommended neighbor base stationsidentified in MOB_SCN-RSP or AAI_SCN-RSP MAC management/control messageand sends a report to the serving BS at the end of the scanning intervalvia mobile scanning interval report (MOB_SCN-REP or AAI_SCN-REP) MACmanagement/control message

The above process/procedure can be applied to a Femto-Aware mobilestation. For a legacy terminal or a Femto-Unaware mobile station, thecell search and cell selection procedures may be in accordance withthose specified in IEEE Std 802.16-2009 and IEEE 802.16m standards andsome transparent optimizations may be made that are outside the scope ofthis proposal.

The above process/procedure is further applicable to the 3GPPLTE-Advanced technology.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims. The following claims are hereby incorporatedinto the detailed description, with each claim standing on its own as aseparate embodiment.

1. A method performed by a mobile station for discovery and association,of a femto base station (BS) from a plurality of base stations, themethod comprising: selecting a first of the base stations forconsideration for discovery and association by the mobile station;decoding a cell ID sequence associated with the first base station; andselecting a further one of the base stations for consideration fordiscovery responsive to determining, based on the cell ID sequence, thatthe first base station is a macro base station and the mobile station isconfigured to select a femto base station over a macro base station; andselecting the further one of the base stations for consideration fordiscovery based on determining, based on the cell ID sequence, that thefirst base station is a Closed Subscriber Group (CSG) femto basestation, and the mobile station is not a member of a CSG associated withthe femto base station, wherein the determination that the mobilestation is not a member of the CSG associated with the femto basestation comprises: decoding an Operator-ID associated with the CSG femtobase station; associating an operator with the CSG femto base stationbased on the Operator-ID; and determining that the mobile station is nota subscriber to the operator associated with the CSG femto base station;wherein, responsive to determining that the first base station is amacro base station and the mobile station is configured to select afemto base station over a macro base station, refraining fromdetermining that the first base station is a CSG femto base station. 2.The method of claim 1, wherein: the femto base station operates at alower power level than the macro base station; the femto base stationoperates with less processing capability than the macro base station;and a density of femto base stations in a geographic area is greaterthan a density of macro base stations in the geographic area.
 3. Themethod of claim 1, wherein selecting the first of the base stationsincludes: measuring a received signal strength associated with the firstone of the plurality of base stations; and based on a determination thatthe received signal strength of the first one of the plurality of basestations is unsuitable, selecting the further base station forconsideration.
 4. The method of claim 1, wherein decoding theOperator-ID comprises decoding the Operator-ID from a SecondarySuperframe Header (S-SFH) sub-packet 2 (SP2) in accordance with anInstitute for Electrical and Electronics Engineers (IEEE) 802.16standard.
 5. The method of claim 1, wherein determining that the firstbase station is a CSG femto base station, and the mobile station is nota member of the CSG associated with the femto base station furthercomprises: decoding a BS-ID associated with the CSG femto base station;maintaining a white list local to the mobile station, the white listidentifying one or more CSG femto BS-IDs associated with authorized CSGfemto base stations to which the mobile station has access; based on acomparison of the BS-ID to the white list, determining that the mobilestation is not authorized to access the CSG femto base station; andbased on the determination that the mobile station is not authorized toaccess the CSG femto base station, selecting the further base stationfor consideration.
 6. The method of claim 5, wherein the BS-ID is 48bits in length having 24 most significant bits (MSBs) and 24 leastsignificant bits (LSBs), wherein the 24 MSBs contain the Operator-ID andwherein the method further comprises decoding the 24 LSBs from SecondarySuperframe Header (S-SFH) sub-packets in accordance with an Institutefor Electrical and Electronics Engineers (IEEE) 802.16 standard.
 7. Themethod of claim 5, wherein the comparison is based on less than 48 bits.8. A mobile station to operate in a wireless packet-carrying network,the mobile station comprising: a physical layer circuit to receive anddecode a cell ID sequence associated with the first one of a pluralityof base stations; a MAC layer circuit coupled to the physical layercircuit, to receive and decode a BS-ID; and a processing circuit coupledto the physical layer circuit, to select a first of the base stationsfor consideration for discovery and association by the mobile station,and to: select a further one of the base stations for consideration fordiscovery responsive to determining, based on the cell ID sequence, thatthe first base station is a macro base station and the mobile station isconfigured to select a femto base station over a macro base station; andselect the further one of the base stations for consideration fordiscovery based on determining, based on the cell ID sequence, that thefirst base station is a Closed Subscriber Group (CSG) femto basestation, and the mobile station is not a member of a CSG associated withthe femto base station, wherein the determination that the mobilestation is not a member of the CSG associated with the femto basestation comprises: decoding an Operator-ID associated with the CSG femtobase station; associating an operator with the CSG femto base stationbased on the Operator-ID; and determining that the mobile station is nota subscriber to the operator associated with the CSG femto base station;wherein, responsive to determining that the first base station is amacro base station and the mobile station is configured to select afemto base station over a macro base station, refraining fromdetermining that the first base station is a CSG femto base station. 9.The mobile station of claim 8, wherein the processing circuit furthermaintains a white list local to the mobile station, the white listidentifying one or more CSG femto BS-IDs associated with authorized CSGfemto base stations to which the mobile station has access; and whereinthe processing circuit is configured to determine that the first basestation is a CSG femto base station, and the mobile station is not amember of the CSG associated with the femto base station by at least:decoding a BS-ID associated with the CSG femto base station, anddetermining based on a comparison of the BS-ID to the CSG femto BS-IDsin the white list, that the mobile station is not authorized to accessthe CSG femto base station, to select the further base station forconsideration.
 10. The mobile station of claim 8, wherein: the femtobase station operates at a lower power level than the macro basestation; the femto base station operates with less processing capabilitythan the macro base station; and a density of femto base stations in ageographic area is greater than a density of macro base stations in thegeographic area.
 11. The mobile station of claim 8, wherein theprocessing circuit is configured to select a first of the base stationsfor consideration by: measuring a received signal strength associatedwith the first one of the plurality of base stations; and based on adetermination that the received signal strength of the first one of theplurality of base stations is unsuitable, selecting the further basestation for consideration.
 12. The mobile station of claim 8, whereinthe processing circuit is configured to decode the Operator-ID from aSecondary Superframe Header (S-SFH) sub-packet 2 (SP2) in accordancewith an Institute for Electrical and Electronics Engineers (IEEE) 802.16standard.
 13. The mobile station of claim 12, wherein the BS-ID is 48bits in length having 24 most significant bits (MSBs) and 24 leastsignificant bits (LSBs), wherein the 24 MSBs contain the Operator-ID andthe processing circuit is configured to decode the 24 LSBs fromSecondary Superframe Header (S-SFH) sub-packets in accordance with anInstitute for Electrical and Electronics Engineers (IEEE) 802.16standard.
 14. The mobile station of claim 12, wherein the comparison isbased on less than 48 bits.